The present invention relates generally to an x-ray anode, and more particularly, to an x-ray anode having graded coefficient of thermal expansion material layers between the substrate and the target material.
There is a desire in the medical imaging industry to accommodate an increasing customer preference for shorter scan times in Computed tomography (CT) x-ray scanners. In order to accomplish the shorter scan times, the x-ray tube on the CT scanner is rotated around a gantry at increasing speeds and the instantaneous power of the x-ray tube directed at the target material on the anode must be increased to maintain X-ray flux. These two requirements require the target diameter on the anode to be maximized within the allowable design envelope and the anode rotation speed to be increased in order to allow the target to handle the high powers required by the shorter scan times.
As the x-ray tube on the CT scanner is rotated around a gantry at increasing speeds, the Hertzian load on the anode bearings dramatically increases. Also, by increasing instantaneous power on the anode, there is an increase in localized temperature on the target of the anode and an increase in temperature variation across the anode. The current anode materials of construction limit the allowable load and instantaneous power that the anode may be subjected to. Thus, the parameters (target diameter and anode rotation speed) are limited by excessive mass, e.g. formed from a solid target material, or unacceptable burst strength of the target material on the anode. The excessive mass increases the load upon the anode bearings; therefore it is desirous to have lighter target material. The localized temperature and temperature variation affects the burst strength, therefore it is desirous to have an anode that is less susceptible to the temperature variations by increasing its strength (burst resistance).
U.S. Pat. No. 6,554,179 teaches a method of reaction brazing a solid target material made from refractory metals to a carbon composite to achieve phase stability between the materials and to achieve high thermal conductivity, which dissipates the localized heat generated on the target material. A slurry coating is applied to graphite or carbon composite containing reactive metal carbides, refactory metal borides, and metal powders to form a layer to which the solid refractory metal alloy can be brazed. The target material is made from solid refractory metal alloys of tungsten (W) or Molybdenum (Mo). The carbonaceous material is preferred to have a matched coefficient of thermal expansion with the target material, otherwise high strains result between the materials during expected temperature excursions. The solid target material adds a significant amount of weight to the anode, thus increasing the overall density of the anode. Furthermore, various methods of making the slurry coating are taught, including methods of applying the slurry to the x-ray anode. Also, heat-treating temperatures and durations are presented for bonding the various refractory metal to the carbonaceous support.
It would therefore be desirable to provide an x-ray anode capable of handling an increased target diameter and anode rotation speeds by designing a lighter weight anode having materials with high strength (burst resistance), high thermal conductivity and reduced strain between the material layers, while ensuring phase stability in the target region over the life of the anode.